The present invention relates to a heterojunction bipolar transistor using a semiconductor layer including silicon, and in particular, relates to measures taken to reduce the driving voltage of the heterojunction bipolar transistor.
Conventionally, a heterojunction bipolar transistor (HBT) has attracted attention as a high-function element. The HBT is a transistor in which the compositions of an emitter and a base are determined to ensure that the band gap of the emitter is larger than that of the base, to thereby substantially improve the injection efficiency of the emitter and thus improve the characteristics of the transistor. The HBT, which is particularly excellent in high-frequency characteristics, is now gradually finding applications as devices in microwave/millimeter wave high frequency bands. The HBT was conventionally fabricated using a combination of GaAs and AlGaAs that are III–V group compound semiconductors, and the like. In recent years, a SiGe HBT that uses SiGe, which has a band gap smaller than Si, as a base layer is under vigorous research and development.
The SiGe HBT utilizes the fact that the band gap of Ge (0.66 eV at room temperature) is smaller than the band gap of Si (1.12 eV at room temperature) and therefore the band gap of SiGe mixed crystal is smaller than that of Si. By using a Si layer as the emitter region and a SiGe layer as the base region, to ensure that the band gap of the base region is smaller than that of the emitter region, the resultant HBT can be driven at a voltage lower than the driving voltage of a Si homojunction bipolar transistor (about 0.7 V). The driving voltage of a bipolar transistor as used herein refers to a voltage in the state where the base-emitter voltage is equal to a base-emitter diffusion potential in an active region of the bipolar transistor. To state more specifically, in an NPN bipolar transistor, it is possible to increase the energy gap at the valence bands edges of the emitter layer and the base layer to some extent to suppress injection of holes from the base layer to the emitter layer, and at the same time, reduce the energy gap at the conduction bands edges of the emitter layer and the base layer. In this way, the driving voltage of the transistor can be reduced.
In the HBT, also, it is possible to provide a base region having a gradient composition in which the Ge content gradually increases from the emitter region toward the collector region, so that the band gap of the base region is gradually reduced from the emitter region toward the collector region. Under an electric field generated by this gradient. composition, traveling of carriers injected in the base layer is accelerated causing drifting. By this drift electric field, the carriers travel faster in the base region than they travel due to diffusion. This shortens the traveling time in the base region, and thus can improve the cutoff frequency (fT).
However, since the lattice constant of Ge (5.65 Å) is different from the lattice constant of Si (5.43 Å), if the Ge content is large, dislocation is generated due to strain caused by the difference in lattice constant. This deteriorates the electric characteristics. In short, in order to further facilitate low-voltage driving, the Ge content of the SiGe layer must be increased. However, as the Ge content of the SiGe layer is larger, the difference in lattice constant is greater between the SiGe layer and the Si layer. The Ge content therefore has an upper limit. To overcome this problem, attention is paid to the fact that the lattice constant of C crystal is smaller than the lattice constant of Si crystal. That is, SiGeC mixed crystal made of a SiGe layer containing C can reduce the strain due to the difference in lattice constant (L. D. Lanzerotti, A. St. Amour, C. W. Liu, J. C. Strum, J. K. Watanabe and N. D. Theodore, IEEE Electron Device Letters, Vol. 17, No. 7, p. 334 (1996)). Therefore, a HBT utilizing heterojunction between a Si layer and a SiGeC layer may be considered. This HBT however has a problem that an impurity contained in the base region diffuses into the collector region during heat treatment, forming a so-called parasitic barrier between the base and the collector (J. W. Slotboom, G. Streutker, A. Pruijmboom and D. J. Gravesteijn, IEEE Electron Device Letters 12, p. 486 (1991)). The formation of a parasitic barrier causes reduction of a gain (β) and deterioration of an early voltage Va and the cutoff frequency fT. To solve this problem, an undoped spacer layer may be interposed between the base and the collector (E. J. Prinz, P. M. Garone, P. V. Schwartz, X. Xiano and J. C. Strum, IEDM Technology Digital, p. 853 (1991)). C has an effect of suppressing impurity diffusion (L. D. Lanzerotti, J. C. Strum, E. Stach, R. Hull, T. Buyuklimanli and C. Magee, Applied Physics Letters 70 (23) p. 3125 (1997)). With this effect, it is expected that the profile of boron as a p-type impurity in the base region can be retained and thus the characteristics such as the early voltage Va and the cutoff frequency fT can be improved.
However, the conventional SiGeC HBT utilizing SiGeC/Si heterojunction has the following problems.
When it is attempted to further reduce the band gap of the SiGeC layer as the base region of a SiGeC HBT for the purpose of further improving the gain, for example, the Ge content of the SiGeC layer must be further increased. As described above, with increase of the Ge content, lattice strain occurs, and to reduce the lattice strain, the content of C may be increased. However, according to experiments carried out by the present inventors, it has been found that the high-frequency characteristics of a HBT deteriorate when the C content is increased. For example, in a HBT using a SiGeC layer having a C content of 0.8% or more as the base region, the n value of a base current is about 2. Hereinafter, the results of the experiments carried out by the present inventors will be described.
FIGS. 8A and 8B are Gummel plots of a SiGe0.268 HBT and a SiGe0.268C0.009 HBT, respectively. FIGS. 9A and 9B are views showing the gains (β) of the SiGe0.268 HBT and the SiGe0.268C0.0091 HBT, respectively. Note that the expression of “the SiGe0.268 HBT”, “SiGe0.268C0.0091 HBT”, and the like as used herein indicates that the mole fraction of Si is a value obtained by subtracting the total content of the other materials (Ge, C, and the like) from 1.
As is found from comparison between FIGS. 8A and 8B, the n value (gradient) of a base current Ib of the SiGe0.268 C0.0091 HBT is significantly inferior compared with the n value of the base current Ib of the SiGe0.268 HBT. Also, as is found from comparison between FIGS. 9A and 9B, the gain β of the SiGe0.268C0.0091 HBT is only 50 at maximum, which is inferior compared with the gain β of 400 of the SiGe0.268 HBT at maximum. The reason is considered as follows. The n value deteriorates because a recombination current increases as the C content is close to 1% in the SiGeC HBT, and with the deterioration of the n value, the gain β decreases.
FIG. 10 is a view for examining fitting between the measurement results of the forward current-voltage characteristics in the emitter-base diode characteristics of the SiGe0.268 HBT and the SiGe0.268C0.0091 HBT and the calculation results of the sum of a recombination current and a diffusion current of electrons. In FIG. 10, the calculated results of the sum of the recombination current and the diffusion current of electrons of the diode are fitted with the measurement results using a recombination lifetime (τr) in an emitter-base depletion layer as a parameter. As is found from the results of the diode characteristics, while the recombination lifetime is about 100 nsec in a SiGeC layer having a C content of 0% (that is, SiGe layer), it is about 400 psec in a SiGeC layer having a C content of 0.91%. It is therefore considered that as the C content is close to 1%, the recombination lifetime significantly decreases, which greatly increases the recombination current. As a result, the characteristics deteriorate.
FIGS. 11A and 11B are views showing the results of simulation of the Gummel plot and the gain, respectively, obtained by varying the recombination lifetime in the base region of a SiGe0.268 HBT, which includes Ge uniformly, from 1×10−5 sec to ×10−9 sec. As is found from FIG. 11A, as the recombination lifetime is shorter, the recombination current of the base current greatly increases, causing deterioration of the n value, while the collector current is not influenced so much. As is found from FIG. 11B, since the recombination current of the base current increases as the recombination lifetime is shorter as described above, the gain β significantly decreases. That is, a short recombination lifetime causes deterioration of the transistor characteristics.
One reason why the recombination lifetime is shortened when the C content of the SiGeC HBT is large is that in SiGeC crystal having a high C content, the amount of C existing at interstitial positions of the crystal is large. The C existing at interstitial positions constitutes a recombination level, and this increases the recombination current.